When North Korea conducted its third nuclear explosion on February 12, the global nuclear police, the Comprehensive Test Ban Treaty Organization, detected it immediately and soon afterward released its evidence. Most people probably know that the CTBTO uses seismometers to detect these forbidden explosions, but it also relies on instruments that monitor infrasound: sound that is too low in frequency for us to hear. Stations in Japan and China Russia recorded the superdeep bass roar of the bomb as it arrived from North Korea through the air, not the ground.

Three days later came the big meteor explosion in Russia. That was high in the atmosphere, not underground, and it gave infrasound its chance to shine. The tremendous shock wave that shattered the windows of Chelyabinsk spread over the world and was recorded by 17 of the CTBTO's 45 infrasound stations, even one in Antarctica. (The CTBTO has issued the record from the Antarctic station, sped up 135 times to make it audible.)

Atom bombs and meteors are just part of the subject, though—all sorts of things create infrasound, both natural and human. Natural sources include volcanoes, wind tumbling over mountain ranges, heavy surf, auroras, tornadoes, landslides, earthquakes and of course meteors. Human sources include vehicle traffic of all kinds, rocket launches, sonic booms, quarry blasts, military bombardments and of course nuclear explosions.

The lower limit of human hearing—the deepest bass notes of thunder or big church organ pipes—is a frequency of about 20 hertz, and infrasound goes from about 20 Hz down to perhaps 0.001 Hz, or one cycle in a thousand seconds. Anything slower than that is more meteorology than acoustics! Infrasound includes ordinary sound-type pressure waves as well as gravity waves, which are like ripples in a pond only in the atmosphere.

Infrasound waves are called microbaroms in the literature. It takes special large instruments to detect them, like the one shown in the photo. The flowerlike apparatus is a set of thin pipes that minimizes the noise of wind, arranged around a pressure sensor. The best instruments are networked arrays of these, scattered over an area measured in acres. Even so, infrasound arrays are put in remote places to avoid the noises of civilization. In California, UC San Diego has a strong infrasound program that includes research arrays near Temecula, Santa Margarita and Chico. It also runs one of the official CTBTO infrasound sites, near Palm Springs.

Science awoke to the potential of infrasound back in 1883, when the enormous eruption of Krakatoa sent waves through the air that registered on barometers around the world. But I first read about infrasound in Robert Heinlein's 1941 novel The Day After Tomorrow (also known as Sixth Column), in which "subsonic" vibrations were used to induce feelings of terror in an enemy delegation. If I recall correctly, the frequency used was 14 Hz. There is something uncanny about sound that can't be heard, and there is a strong flavor of paranoia around the subject in some circles. High levels of infrasound do have health effects, particularly because the vibrations rattle the whole organism, not just the ears. More benignly, elephants and whales are known to perceive and produce infrasound. And one intriguing line of research suggests that migratory birds use infrasound for navigation as a sonic map of the regional landscape.

The Comprehensive Test Ban Treaty gave a big boost to infrasound research. In addition to its well-known seismometers and radiation monitors, the CTBTO's worldwide network of sensors includes a planned total of 60 infrasound stations, of which 45 are working today. (A network of hydrophones, to monitor the seas, is the fourth component.) The United States Senate has still not ratified the treaty, although Chad did just this month. President Clinton signed the treaty on September 24, 1996, but ever since then the Senate has failed to do its part. It would be nice if it did, and soon.

Andrew Alden earned his geology degree at the University of New Hampshire and moved back to the Bay Area to work at the U.S. Geological Survey for six years. He has written on geology for About.com since its founding in 1997. In 2007, he started the Oakland Geology blog, which won recognition as "Best of the East Bay" from the East Bay Express in 2010. In writing about geology in the Bay Area and surroundings, he hopes to share some of the useful and pleasurable insights that geologists give us—not just facts about the deep past, but an attitude that might be called the deep present.
Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.